1. Field of the Invention
The invention relates to a photothermographic silver halide material and method for producing in such a material improved fog stability on shelf aging by incorporating vinyl sulfones and/or .beta.-halo sulfones.
2. Background of the Art
Silver halide photothermographic imaging materials, often referred to as "dry silver" compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
Photothermographic emulsions, in a manner similar to photographic emulsions and other light sensitive systems, tend to suffer from fog. Fog is spurious image density which appears in unexposed areas of the element and is often reported in semitometric results as D.sub.min.
Traditionally, photothermographic materials have suffered from fog instability on shelf aging. The fog level rises steadily as the materials reach extended age such as one year at room temperature (ambient conditions). Adding to the difficulty of fog control on shelf aging is the fact that the developer is incorporated in the photothermographic element, which is not the case in most silver halide photographic systems. The need for shelf life extenders in photothermographic elements is therefore considered to be very important.
The fog level of freshly prepared photothermographic materials will be referred to as initial fog. A great deal of effort has been directed towards minimizing the initial fog and stabilizing the fog level on shelf aging. Mercuric salts are described as antifoggants in U.S. Pat. No. 3,589,903. Fog reduction has been described for organic carboxylic acids such as benzoic and phthalic acids in U.S. Pat. No. 4,152,160, for benzoyl benzoic acid compounds in U.S. Pat. No. 4,784,939, for indane or tetralin carboxylic acids in U.S. Pat. No. 4,569,906, for dicarboxylic acids in U.S. Pat. No. 4,820,617 and for heteroaromatic carboxylic acids in U.S. Pat. No. 4,626,500. Halogenated compounds have also been shown to be strong antifoggants and are described in U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946 and 3,955,982. Halogen molecules or halogen molecules associated with a heteroatom ring are also useful antifoggants and are described in U.S. Pat. No. 5,028,523. However, these compounds individually or in combination were not found to produce sufficient fog stability on shelf life aging of the photothermographic element.
Vinyl sulfones, and to a lesser extent .beta.-halo sulfones, have been used extensively in photographic constructions as gelatin hardeners or crosslinking agents. In order to crosslink the gelatin, two or more vinyl sulfonyl groups or two or more .beta.-halo sulfonyl groups are attached to the same molecule by a linking group. Examples of these gelatin hardening agents are found in U.S. Pat. Nos. 3,839,042, 3,841,872 and 3,957,882.
Many additional patents on gelatin hardening by vinyl sulfones and .beta.-halo sulfones have appeared covering modifications to the linking group. The modifications include the addition of water solubilizing groups as in U.S. Pat. Nos. 4,173,481, 4,142,897 and 4,323,646 and the incorporation of a heteroatom in U.S. Pat. Nos. 3,490,911, 3,642,486 and 4,134,770. Additional patents have been issued covering three or more vinyl groups on a compound (U.S. Pat. No. 4,088,495) and the use of polymeric vinyl sulfone and .beta.-halo sulfones as in U.S. Pat. No. 4,956,270 and Japanese Patent Application J63123039-A.
Vinyl sulfones and .beta.-halo sulfones have received less attention in photothermographic systems. In the photothermographic systems, they have again been used to harden or crosslink a hydrophilic binder. They are referred to as hardeners for hydrophilic binders in Japanese Patent Application JO 3114043A and JP 61018942A and in a thermal transfer system covered by Japanese Patent Application J6 2177546A and in a color dye diffusion system outlined in U.S. Pat. No. 4,840,882. Our system varies greatly from these in that the vinyl sulfones and .beta.-halo sulfones are used with hydrophobic binders and no hardening or crosslinking is observed.